1. Field of the Invention
The present invention relates generally to power regulator, governor and control devices. The present invention applies specifically to situations where less than the full power operation of a powered device is desired. The invention relates more specifically to devices which translate the discrete operation of mechanical controls into the efficient manipulation of power in a power operated device.
2. Description of the Related Art
Innumerable devices in society today consume power as they operate or function and perform work. This power is delivered to these devices in a great variety of forms. Power may, for example, be delivered to an operating device in the form of a pressurized liquid or gas. Once present within the device this pressurized liquid or gas can be utilized to do work. Power may also be delivered to an operating device by the conduction of electricity in a manner that results in a flow of current, the establishment of an electromotive force, or the establishment of a magnetomotive force. This conduction of electricity may be either direct or alternating in nature. Once present within the device this flow of current, electromotive force, or magnetomotive force can also be utilized to do work. But whether power is delivered by hydraulics, pneumatics, vacuum pressures, direct currents, alternating currents, static voltages, varying voltages, magnetics, or some other means for doing work, this delivery is always going to involve problems of efficiency and control.
The controlled delivery of power, regardless of the form that the power is delivered in, usually involves a number of common concerns. First is the manner in which power delivery is initiated. Second is simply the amount of power to be delivered, and the limitations on the power level as defined by the consumptive capacity of the device. Third is the manner in which a transition from one power level to another is achieved. Fourth is the manner in which a controlled limiting of power is achieved. The efficient use of power implies that a refined means for dealing with each of the above four concerns be implemented. The present invention seeks to address these concerns.
Because the delivery of power generally deals with the common concerns described above, regardless of the medium by which power is delivered, the discussion of a specific medium and system for the delivery of power can be easily translated into an analogous discussion involving a distinct medium and system. Thus while the present discussion will address the manifestations of the above described concerns primarily in the governance of electrical current, it should be apparent that the problems and solutions identified with regard to electrical current power systems, also identify analogous problems and solutions in areas where the delivery of power is achieved by other means.
The present discussion will therefore focus on the governance of electrical current, and will specifically address the governance of direct current to the motor of a motor driven vehicle. Electrically powered vehicles face each of the four concerns identified above. The transition from a stationary condition to a moving condition is important. The speed of the motion is important as is the maximum current flow to the motor. The acceleration and deceleration of the motion is important. And finally the controlled transition from a moving condition to a stationary condition is important.
The most common method of regulating the speed of an electric motor, and thus the speed of an electric vehicle, typically involves placing a variable resistor or a sequence of discrete resistors in series with the windings of the electric motor. While this method does provide speed control of the motor it has a number of distinct disadvantages.
First, the power drawn from the battery in such an arrangement is not efficiently reduced in direct proportion to the speed of the motor when the speed is reduced. This is because a portion of the power is dissipated through the resistors rather than entirely through the motor. The same current drain occurs on the battery whether the motor is run at high speed or low speed. The only change is in the relative distribution of the load between the resistors and the motor windings.
A second disadvantage, which is a by product of the first, is that the power dissipated through the resistors is given off as heat, which in addition to being a waste of energy, can create heat transfer problems in some applications.
A third disadvantage results from the inaccuracies associated with physically controlling the condition of a variable resistor or the selection of an array of discrete resistors. If the speed adjustment is accomplished by means of a variable resistor or potentiometer, then some physical movement of one contact across a resistive coil or surface will be required to provide the proper resistive input to a motor control circuit. If a series of discrete resistors is utilized, then a similar contact will have to be switched from one resistor to another, thereby presenting a resistance of a given value to the input of the motor control circuitry. In either case, there is seldom a smooth transition from one resistance value to another as the device is physically manipulated by a foot pedal or a hand control.
Electrically powered vehicles enjoy the advantage of being able to rapidly alter the current flow to the motor drive system and thus rapidly alter the motion of the vehicle. This advantage can become a disadvantage when such rapid changes result in problems associated with the control, stability, safety, and integrity of the vehicle.
Electrically powered vehicles such as forklifts and hand manipulated "walkies" are typically utilized within confined spaces and in direct contact with workers who may or may not be in control of the vehicle. These conditions raise additional safety concerns for both the vehicle and the workers. If the vehicle were to encounter a stationary, non-movable obstruction, the load placed upon the motor could draw a current in excess of the capacity of the motor or the control system. Some means of rapidly correcting or preventing such an overcurrent situation would be desirable.
Some electrically powered cargo moving vehicles are controlled by operators who walk behind the vehicle rather than ride on the vehicle itself. These "walkies" can potentially pin the operator between the vehicle and a stationary object. A safety switch known as a "belly" switch is typically incorporated into the hand controls of such vehicles and is designed to reverse the direction of he vehicle when a dangerous situation occurs. It would also be desirable to preserve the function of such safety devices in the control of the electric motor as well.
There is additionally no easy way of incorporating other velocity, acceleration, deceleration, or safety control means into the motor control circuitry. In many situations, there are other factors that can and should effect the function of the electric motor. Attempting to integrate all of these other control means in parallel with the primary speed control means of a variable resistor or a sequence of discrete resistors can be complicated if not impossible.
Attempts at solutions to the problems identified above have sometimes utilized solid state switching devices to control and regulate the current flow to the motor windings. One application of such a solid state device, a metal oxide semiconductor field effect transistor or MOSFET, is disclosed in U.S. Pat. No. 5,029,229. The power control circuit disclosed therein utilizes MOSFET devices to toggle on and off a relatively large current flow into a DC electric motor circuit.
While circuits of the type disclosed in the above referenced patent do provide an efficient way of controlling the current to a DC electric motor, they may still rely upon a variable resistance or variable voltage input for their own control or regulation. Such circuits solve the first two of the disadvantages identified above, but do not rectify the additional disadvantages that pertain to the inaccuracies associated with the standard variable resistor or multiple resistor input, and the inability to input parallel signals to control acceleration, deceleration, and sudden changes in direction, as well as overcurrent and safety conditions.